parametric study of a phase separator used in a/c ... · cyclone phase separator is simple, compact...
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Research Journal of Engineering Sciences ___________________________________________ ISSN 2278 – 9472
Vol. 1(3), 15-21, Sept. (2012) Res. J. Engineering Sci.
International Science Congress Association 15
Parametric Study of a Phase Separator used in A/C Automotive System
using CFD Tool
Thundil Karuppa Raj R.*, Alen Jose, Ajo John Thomas and Mathew James
School of Mechanical and Building Sciences, VIT University, Vellore-632014, Tamil Nadu, INDIA
Available online at: www.isca.in Received 24th July 2012, revised 15th August 2012, accepted 21st August 2012
Abstract
The air conditioning system used in automobiles uses oil which lubricates the moving part of the compressor. Excess
presence of the lubricant oil in the compressor will cause a hindrance to flow inside the condenser and evaporator which
will affect the heat transfer efficiency of the system. The oil is separated at the downstream of the compressor using
centrifugal/ impinging on the refrigerant gas flow. The oil exists in the form of droplets in a secondary phase. The main
requirement for a good design of the phase separator is to have maximum oil separation with minimum pressure drop of the
refrigerant gas. The numerical simulation is carried out using Multiphase Mixture model.
Keywords: Phase separator, multiphase flow, mixture models.
Introduction
Air conditioning compressor is an integral component for an air
conditioning system; when it begins reciprocate, lubricant will
be very pivotal to protect equipment. But when refrigerant is
exhausted from compressor, a little of lubricating oil will be
taken away from it and will accumulate inside the compressor
eventually. Therefore, an oil-gas separator need be collocated to
compressor, which could separate lubricating oil from
refrigerant and then lubricating oil would flow back compressor.
For the separator construction, the swirling motion is brought
about by designing the inlet in such a manner that it forces the
gas to enter the unit on a tangent to the inner body wall1.
There are many different varieties of oil-gas separator available
in the market. One such variety is the cyclone phase separator
works on the principle of double vortex. The oil-gas mixture is
injected into the cylinder using a pipe which is held in
tangentially to the separator body is shown in figure 1. As the
gas swirls, it moves axially downwards in the outer part of the
separation space. In the conical part of the cyclone, the gas is
slowly forced into the inner region of the cyclone, where the
axial movement is upwardly directed and the downwardly
directed axial flow takes oil particles along with it. This flow
pattern is often referred to as a ‘double vortex’: an outer vortex
with downwardly directed axial flow and an inner one with
upwardly directed flow.
Cyclone phase separator is simple, compact with less weight,
and has low capital and operational costs. The Cyclone Phase
has a wide variety of potential applications, varying from only
partial separation to a complete phase separation2. The analysis
of fluid flow and particle motion in a cyclone is very
complicated. The aerodynamics inside the cyclone create a
complex two-phase, three-dimensional, turbulent swirling flow
with a confined outer free vortex (irrotational flow) and a low-
pressure, highly turbulent inner forced vortex (solid body
rotation). The transfer of fluid from the outer vortex to the inner
vortex apparently begins below the bottom of the exit tube and
continues down into the cone along the natural length of the
vortex of a cyclone3. The length of the inner vortex core is also
referred to as the cyclone effective length, which does not
necessarily reach the bottom of the cyclone3. Particle/ Fluid
collection in the cyclone is due to the induced inertia force
resulting in radial migration of particles suspended in the
swirling gas to the walls and down the conical section to the
dust outlet and the gas exits through the vortex finder. Flow near
the cyclone wall is assumed to be laminar, although it is usually
somewhat turbulent4.
Figure- 1
Schematic working diagram of a two- phase cyclone
separator
Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472
Vol. 1(3), 15-21, Sept. (2012) Res. J. Engineering Sci.
International Science Congress Association 16
The main objective of this paper is to carry out a parametric
study on various parameters which affects the performance of
the cyclone separator used in automotive air conditioning
systems with minimum losses. The factor which determines the
separator performance is the amount of gas (refrigerant)
separated from the mixture. The flow inside the separator is
highly turbulent multiphase flow in which the refrigerant
(CO2/R134a) to be in the primary phase and the lubricant (Poly
Alkaline Glycol) which is in the secondary phase. Flow
characteristics for different geometric configurations were
analyzed numerically.
Methodology
Formulation of the Model: Building CFD Model: The
geometrical models (surface modeling) are designed using
CATIA V5 based on the parameters given in the in the figure 2
form literature5.
The gird generated for the present study is purely tetrahedral
meshes. The analysis for hexahedral mesh is also carried out but
it is showing a significant variation form the values adopted
form literature. Fine control over the tetrahedral mesh near the
mixture inlet and oil outlet will help to capture the boundary
layers more effectively. The tetrahedral mesh generated is
shown in figure 3.
Figure -2
Schematic working diagram of a 2 phase cyclone separator
(a)-(b)
Figure- 3
Meshed model of the phase separator in discretized using ICEM CFD
Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472
Vol. 1(3), 15-21, Sept. (2012) Res. J. Engineering Sci.
International Science Congress Association 17
Computational Fluid Models for Multiphase flow: A
multiphase system is defined as a mixture of phases of solid,
liquid and gas. Multiphase flows are classified according to the
nature of system: dispersed flows (particle or droplets in liquid
or gas, bubbles in liquid), separated flows (annular flows in
vertical pipes, stratified flows in horizontal pipes) and
transitional flows, which are combinations of the other two
classes 5
. In this work we are focusing on multiphase flows
where the secondary phases cannot be ignored due to their
influence on the fluid dynamic behavior of the mixture and
partly also due to their importance for the process studied.
Depending on the strength of the coupling between the phases,
different modeling approaches are suggested. They can be
classified into homogeneous flow models, mixture models and
multiphase models. Combinations of these are possible, too. In
most models, each phase is treated as an interpenetrating
continuum with a volume fraction parameter, which is
analogous to the porosity assigned to a fluid phase in flow
through a porous medium6.
In all multiphase models, the main difficulties are due to the
interfaces between the phases and the discontinuities associated
to them7. The formulation of the constitutive equations is the
greatest difficulty when developing a multiphase model for a
practical application8.
The available models for defining a multiphase flow is Discrete
Phase Model, Eulerian Phase Model and Mixture model. In
Discrete Phase Model (DPM) in which trajectories of
particles/droplets are computed in a Lagrangian frame and have
the capabilities of coupling these particles to the gas phase in
the Eulerian frame. In its standard form the DPM model does
not account for volume fractions of the discrete phase
particles9,10
.
Considering the computation expenses and time, the discrete
phase model, Eulerian Phase model is not considered for the
present study. In addition to that the mixture multiphase model
can give satisfactory results with less error and will account
more accurately for the collision and merging effects of the oil
droplets in the primary gas phase.
Multiphase Mixture Model: The continuity equation for the
mixture, the momentum equation for the mixture, the energy
equation for the mixture, the volume fraction equation for the
secondary phases, algebraic expressions for the relative
velocities (if the phases are moving at different velocities)11
.
Assumptions: i. Refrigerant and oil flows are incompressible
within the oil separator; ii. There is no inter-phase mass transfer
between refrigerant and oil; iii. Oil-droplets have an average
diameter, which can be calculated in a weighted-average
manner; iv. The mixture flow is isothermal, i.e., refrigerant and
oil properties can be calculated at an average discharge
pressure and temperature conditions; v. Steady state analysis is
performed for the reported calculations; vi. Gravity forces are
acting downward along the vertical axis of the separator body,
with the regular magnitude g=9.81 m/s2
Boundary Conditions: Mixture Inlet: The inlet boundary
condition involves velocity inlet along with turbulent intensity,
hydraulic diameter. The velocity of air at the inlet is varied from
1 m/s to 10m/s. The volume fraction initially is taken as 0.007
and varied till 0.025.
Oil Outlet: The gas and oil outlet is designated as a pressure
outlet with the operating pressure of 101325Pa.
Wall: Wall boundary conditions are enforced on all faces
bounding the flow. Adiabatic, no-slip boundary conditions are
applied at the walls.
Multiphase- Fluid: This boundary condition is applied to all
volumes of the geometry for the fluid to flow through. Initially
the separator has 100% refrigerant inside. The direction of the
flow is along the axial direction.
Turbulence Model: The RNG k -ε model is selected for the
present work because the effect of swirl on turbulence is
included in the RNG model, enhancing accuracy for swirling
flows.
Flow Solver: The phase-coupled SIMPLE algorithm with
pressure spatial discretization as PRESTO!, is selected for
pressure-velocity coupling while the second order scheme is
employed for the discretization of the remaining equations
except with volume fraction which is discretized according to
First order upwind scheme
Separator Performance: Separator performance is defined by
following parameters based on the steady state CFD calculation.
The main aim of this work is to have maximum separator
efficiency of gas and oil form the inlet . The various parameters
that specify these are.
Gas separation Efficiency: ηgas =m2gas/m1gas
Liquid Separation Efficiency: ηliq =m3liq/m1liq
Where: m2gas= mass flow rate of refrigerant gas exiting through
the top outlet, m1gas= mass flow rate of refrigerant entering the
system through the inlet, m3liq= mass flow rate of oil droplets
exiting through the bottom outlet, m1liq = mass flow rate of oil
droplet entering the device.
Validation: Validation of the phase separator is carried out
based on the different cases by Tiberiu Barbat, Kanwal Bhatia
and Srinivas Pitla9
is shown in table 1. The separator
performance is numerically analyzed under nominal operating
conditions (1 m/s inlet velocity, 0.7% liquid volume fraction at
the inlet, droplet diameter dp = 10µm) and compared with the
results predicted in the literature 10. The separator efficiencies
Research Journal of Engineering Sciences___________
Vol. 1(3), 15-21, Sept. (2012)
International Science Congress Association
obtained numerically is shown in figure 4. Considering the
separation efficiencies of all the 9 cases maximum deviation is
obtained to be 7.89%.
Table -1
Cases based on Design Parameters
Rbot [mm] Hpipe [mm] Rpipe
Case 1 6 30 2
Case 2 7.5 30 2
Case 3 3.125 30 2
Case 4 6 15 2
Case 5 6 40 2
Case 6 6 30 1
Case 7 6 30 4
Case 8 6 30 2
Case 9 6 30 2
(a)
(b)
Figure – 4
Validation (a), (b)
Results and Discussions
Replacing the Refrigerant CO2 with R134a
CO2 used in the literature is not used in the market in an
extensive manner12
. The next stage in the study is to replace
CO2 with conventionally used refrigerant. It is observed form
the literature by J. Steven Brown and Samuel F. Yana
020406080
100
Ca
se-1
Ca
se-2
Ca
se-3
Ca
se-4
Ca
se-5
Ca
se-6
Ca
se-7
Ga
s S
ep
ara
tio
n E
ffic
en
cy
(%)
Cases
Present Work Literature [10]
020406080
100
Ca
se-1
Ca
se-2
Ca
se-3
Ca
se-4
Ca
se-5
Ca
se-6
Ca
se-7
Oil
Se
pa
rati
on
Eff
ice
incy
(%)
Cases
Present Work Literature [10]
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International Science Congress Association
igure 4. Considering the
separation efficiencies of all the 9 cases maximum deviation is
es based on Design Parameters10
[mm] Rout
[mm]
2 1
2 1
2 1
2 1
2 1
1 1
4 1
2 .5
2 2
with R134a: The refrigerant
used in the literature is not used in the market in an
The next stage in the study is to replace
with conventionally used refrigerant. It is observed form
the literature by J. Steven Brown and Samuel F. Yana-
Motta13
on the performance merits of CO
automotive air conditioners; it is found that th
refrigerating system with R134a performs better compared with
CO2. The standard properties of R134a (1,1,1,2
tetrafluroethane) is adapted from the International Journal of
Refrigeration and inputted into the FLUENT V6 database
The nine cases mentioned in the literature
and results are shown below.
(a)
(b)
Figure-
Variation in Separation efficiencies (a) Shows the variation
in Oil Separation efficiency for CO
the variation in Gas Separation efficiency for CO
R134a
When the refrigerant CO2 is replaced with R134a it is observed
that the tendency in variation of the separation efficiency
remained to be the same. The phase separator with
R134a showed an increase in gas separation efficiency with a
maximum variation of nearly 21 % with that of CO
other hand the oil separation efficiency of R134a has a
maximum drop of nearly 46% compared to that of CO
mainly due to change in the refrigerant properties. CFD studies
on various automotive components have been carried out
0
20
40
60
80
100
120
Ga
s S
ep
ara
tio
n E
ffic
ein
cy (
%)
Cases
CO2 R134a
0
20
40
60
80
100
Oil
Se
pa
rati
on
Eff
ice
incy
(%
)
Cases
CO2
Ca
se-7
Ca
se-8
Ca
se-9
Literature [10]
Ca
se-7
Ca
se-8
Ca
se-9
Literature [10]
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Res. J. Engineering Sci.
18
on the performance merits of CO2 and R134a
automotive air conditioners; it is found that the COP of the
refrigerating system with R134a performs better compared with
. The standard properties of R134a (1,1,1,2-
tetrafluroethane) is adapted from the International Journal of
Refrigeration and inputted into the FLUENT V6 database14
.
es mentioned in the literature 9
are again analyzed
5
Variation in Separation efficiencies (a) Shows the variation
in Oil Separation efficiency for CO2 and R134a (b) Shows
the variation in Gas Separation efficiency for CO2 and
R134a
is replaced with R134a it is observed
that the tendency in variation of the separation efficiency
remained to be the same. The phase separator with refrigerant
R134a showed an increase in gas separation efficiency with a
maximum variation of nearly 21 % with that of CO2. On the
other hand the oil separation efficiency of R134a has a
maximum drop of nearly 46% compared to that of CO2 which is
e to change in the refrigerant properties. CFD studies
on various automotive components have been carried out15-18
Cases
R134a
Cases
Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472
Vol. 1(3), 15-21, Sept. (2012) Res. J. Engineering Sci.
International Science Congress Association 19
Parametric Study and Separator performance: In addition to
the 9 cases mentioned above this work tries to find out the effect
of other two design parameters like the height of the phase
separator and mixture inlet diameter in the separator
performance which extends the total number of cases to 17 and
is shown in table 2. All the above cases are tested under nominal
operating conditions (1 m/s inlet velocity, 0.7% liquid volume
fraction at the inlet, droplet diameter dp = 10µm) which is taken
from the literature9.
The main design parameters and its influence in separator
performance are discussed below.
Radius of gas outlet(R pipe): It is clear from table 3 the gas
separation efficiency considerable increases when the radius of
the pipe increases from 1mm to 2mm. The main reason is as the
size of the pipe increases the pressure drop will be considerably
less which reduces the resistance to the flow of gas. As the
radius is increased further to 4 mm, the gas separation efficiency
increases further. But the chances of flow of the oil to the wrong
gas outlet are more in this case; which explains the decreased oil
separation efficiency.
Table -2
Additional Design Parameters under study Cases Rbot
(mm)
Hpipe
(mm)
Rpipe
(mm)
Rout
(mm)
Htotal
(mm)
Rinlet
(mm)
Case 10 6 30 1 1 40 2
Case 11 6 30 1 1 45 2
Case 12 6 30 1 1 55 2
Case 13 6 30 1 1 60 2
Case 14 6 30 1 1 50 1
Case 15 6 30 1 1 50 1.5
Case 16 6 30 1 1 50 2.5
Case 17 6 30 1 1 50 3
Table -3
Separator performance in varying the radius of the pipe
(R pipe) (mm)
Case R pipe
mm
Gas Separation
Efficiency (%)
Oil Separation
Efficiency (%)
Case 6 1 47.42 46.53
Case 1 2 82.15 23.26
Case 7 4 87.16 14.53
Radius of oil outlet (R out): The dimension of the orifice at the
bottom of the separator is also having a significant influence in
both the separator performances. As the radius of the orifice is
increased it can be perceived that the oil separation efficiency
increases due to reduction in pressure drop in the oil path which
results in more oil separation. This will allow more refrigerant
to pass through the oil outlet that results in decreased gas
separation efficiency. Table 4 shows the increase in oil
separation efficiency and decrease in gas separation efficiency
as the radius of the oil outlet orifice is increased.
Table-4 Separator performance in varying the radius of oil outlet
(R out) (mm)
Case R out
mm
Gas Separation
Efficiency (%)
Oil Separation
Efficiency (%)
Case 8 .5 96.6 9.3
Case 1 1 82.15 23.26
Case 9 2 16.31 74.45
Radius of separator bottom (R bot): This parameter is
primarily responsible for the shape of the phase separator. Table
5 shows that the variation in the separator bottom radius, it can
be observed that nearly 3% variation in both separator
efficiencies. This implies that the variation in radius of the
separator bottom has literally negligible influence in the
separator performance
Radius of mixture inlet (R in): This parameter is primarily
responsible for the amount of mixture which is going into the
phase separator. Observing table 6 it is possible to infer that the
influence of mixture inlet in separator performance is not
significant when compared with the above discussed factors. As
the radius increases more and more refrigerant gas will be
incident into the separator this will result in increased flow of
gas to the gas outlet which explains the slight increase in the gas
separation efficiency.
Table-5
Separator performance in varying the radius of separator
bottom(R bot) (mm)
Case R bot mm Gas Separation
Efficiency (%)
Oil Separation
Efficiency (%)
Case 3 3.125 82.13 25.46
Case 1 6 82.15 23.26
Case 2 7.5 79.68 19.82
Table-6
Separator performance in varying the radius of mixture
inlet (R in) (mm) Case R in
mm
Gas Separation
Efficiency (%)
Oil Separation
Efficiency (%)
Case 14 1 30.23 66.12
Case 15 1.5 36.46 65.98
Case 16 2.5 40.74 62.43
Case 17 3 42.33 61.25
Height of the gas outlet pipe (H pipe): Changing the height of
the gas outlet pipe showed a negligible variation in the separator
performances. This implies that the influence of gas outlet pipe
in the separator performance is not significant and this design
parameter can be anywhere in the range given in table 7.
Table-7
Separator performance in varying the Height of the gas
outlet pipe (H pipe )
Case H pipe
mm
Gas Separation
Efficiency (%)
Oil Separation
Efficiency (%)
Case 4 15 81.87 18.16
Case 1 30 82.15 23.26
Case 5 40 83.25 19.14
Research Journal of Engineering Sciences___________
Vol. 1(3), 15-21, Sept. (2012)
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Overall Height of the pipe (H): The final design parameter
which was considered under the present study is the overall
height of the phase separator. Numerical simulations are carried
out for various values of phase separator height and are shown
in Table 8. It is observed that the gas separation efficiency
remains almost constant as the value of H increases. On
contrary to that the value of oil separation efficiency first
increases up to a certain value of H and then decreases. This is
because as the length of the pipe is more, the chances length of
the vortex inside the separator will be more and as a result, more
oil can get separated inside the phase separator up to a certain
limit.
Table -8
Separator performance in varying the height of the
separator (H) (mm)
Case H
mm
Gas Separation
Efficiency (%)
Case 10 40 52.82
Case 11 45 51.52
Case 12 55 52.3
Case 13 60 53.45
Flow Pattern obtained from CFD Analysis
the velocity contour in a multiphase flow for R134a and PAG
taken along the mid plane of the phase separator. It can be seen
that velocity of R134a is more compared to that of PAG oil. The
velocity is remaining almost constant inside the separator body.
The velocity increases as the oil/gas moves towards the exit.
(a)
(b)
Figure - 6
Case 6 Velocity contours for (a) R134a and (b) PAG
_________________________________________________
International Science Congress Association
The final design parameter
which was considered under the present study is the overall
height of the phase separator. Numerical simulations are carried
out for various values of phase separator height and are shown
8. It is observed that the gas separation efficiency
remains almost constant as the value of H increases. On
contrary to that the value of oil separation efficiency first
increases up to a certain value of H and then decreases. This is
gth of the pipe is more, the chances length of
the vortex inside the separator will be more and as a result, more
oil can get separated inside the phase separator up to a certain
Separator performance in varying the height of the
Oil Separation
Efficiency (%)
40.95
47.13
52.54
38.74
Flow Pattern obtained from CFD Analysis: Figure 6 shows
a multiphase flow for R134a and PAG
taken along the mid plane of the phase separator. It can be seen
that velocity of R134a is more compared to that of PAG oil. The
velocity is remaining almost constant inside the separator body.
the oil/gas moves towards the exit.
Case 6 Velocity contours for (a) R134a and (b) PAG
Figure 7 shows the streamline flow pattern obtained for the
mixture. It is clear that a swirling motion is present inside
phase separator which is responsible for the separation of R134a
and PAG oil.
Figure -
Case 6 Streamline flow of the mixture at 1m/s inlet velocity
Figure 8 displays the Volume fraction of R134a and PAG in the
same axial cut in the nominal operating conditions.
that the PAG volume fraction is very low compared to that of
R134a and PAG oil is found to be towards the oil exit.
(a)
(b)
Figure -
Case 6 Volume fraction for (a) R134a and (b) PAG
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Res. J. Engineering Sci.
20
Figure 7 shows the streamline flow pattern obtained for the
mixture. It is clear that a swirling motion is present inside the
phase separator which is responsible for the separation of R134a
-7
Case 6 Streamline flow of the mixture at 1m/s inlet velocity
Figure 8 displays the Volume fraction of R134a and PAG in the
nominal operating conditions. It is clear
that the PAG volume fraction is very low compared to that of
R134a and PAG oil is found to be towards the oil exit.
-8
Case 6 Volume fraction for (a) R134a and (b) PAG
Research Journal of Engineering Sciences________________________________________________________ ISSN 2278 – 9472
Vol. 1(3), 15-21, Sept. (2012) Res. J. Engineering Sci.
International Science Congress Association 21
Conclusion
In the automotive systems, use of a phase separator after the
compressor can increase the COP of air-conditioning system as
it improves the working of the heat exchanger. A separator’s
performance is analyzed using the oil separation efficiency and
gas separation efficiency.
When the refrigerant CO2 is replaced with commercially
available R134a it is observed that the trend in variation remains
the same. But the oil separation efficiency dropped to nearly 50
%. , though the gas separation efficiency is reported to have an
increase in value. The main reason is because of the change in
properties of refrigerant.
Multiphase flow of different geometrical models, meshes and
numerical models are built and run using parametric journal
files. Numerical simulation shows that among the six design
parameters selected, the radius of the pipe (Rpipe) and Radius of
the oil outlet (Rout) is having a significant influence in the
performance of the phase separator. A slight variation in the
above parameters can drastically alter the oil separation
efficiency and gas separation efficiency. The remaining four
design parameters shows that they are having negligible
influence in the separator performance compared with the above
two parameters.
The oil separation efficiency and gas separation efficiency for a
particular parameter cannot hold a higher value in both cases.
There should be a satisfactory compromise between the two. In
the total 17 design parameters discussed in the paper, it is
observed that Case 6 and Case 12 fall under such a case. Their
separator efficiencies are found to be nearly 50 % in both the
cases which is required.
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